How to show collections in blender 2.8
TL;DR: AltH in the Outliner (thanks @aliasguru).



There are a few ways to hide/show collections in 2.8:


In the Outliner area (by default there's one in the top right), clicking the eye icon toggles visibility of the corresponding collection.
Pressing number-row numbers (123 etc.) will switch to the nth collection (according to the order in the outliner), hiding the others.
Pressing Shift + number will toggle visibility for the nth collection.
There is a collections section in the 3D view > N panel > View tab. This one seems to be the most convenient for toggling visibility on lots of collections quickly, using the click+drag toggle behavior.





Pressing CtrlH in the 3D view brings up a "hide collection" dialog which behaves like like pressing numbers (switches to the clicked collection hiding the others). 
As noted previously, pressing AltH in the Outliner unhides all collections.


Also important to note, that the old hide in viewport/hide in render properties still exist on collections and are completely separate from the visibility set by the controls describe above. Controls for those can be found in the right click menu in the outliner, under Visibility:

 

I'm not really sure why it works this way currently, I wouldn't be surprised if it changes in subsequent releases.